Polymer gels that have a network structure have excellent properties such as water retention capacity and biocompatibility, for which reason there has been a focus on studies in which such gels are embedded in vivo as artificial tissues, materials for regeneration scaffolds, and the like (Non-patent Reference 1). A problem, however, has been that polymer gels cause compressive damage in the tissues surrounding the region in which they are embedded because of the osmotic pressure generated from the difference in concentration between the inside of the gel and the outer environment in water. Furthermore, the decomposition of polymer gels elevates the expansion pressure.
Such expansion pressure is proportional to the square of the polymer concentration constituting the gel, and therefore the effects of expansion become more prominent when the polymer concentration is high. Lowering the concentration of the polymer is an essential solution since crosslinks break by changes over time even if the degree of cross linking is raised to lower the expansion ratio. However, it has been difficult to produce a gel in a short time by conventional polymer gel production processes when the polymer concentration is lowered to a level at which expansion does not create tissue damage. It has also been difficult to control the physical properties since physical properties such as the modulus of elasticity change dramatically when the polymer concentration is low and a gel is formed in regions in the vicinity of the gelation points.